This invention relates generally to techniques for fabricating lightweight pressure vessels and, more particularly, for fabricating single-piece pressure vessels using only composite materials and without using a metal liner.
As is well known, composite materials, or composites, combine at least two constituent materials with different physical properties such that the constituents retain their individual properties in the composite, but also complement each other to provide overall properties unattainable from any of the constituents. Examples of composites in ancient times include the use of chopped straw in brick and the use of resin-soaked cloth tape to embalm Egyptian mummies. In current terminology, the term “composite” usually refers to a combination of fibrous material and a matrix material, such as an epoxy. The fibers provide strength in a selected direction or directions and the matrix binds the structure together and transfers forces between the fibers. By some definitions, composites also include laminar layers of materials and material particles embedded in a matrix. Because the present application is concerned with pressure vessels, the term “composites” or “composite materials” is intended primarily to encompass combinations of fibrous materials, such as graphite (carbon) fibers or glass fibers in a suitable matrix. It will be understood, however, that other lightweight composite structures may be used to the same effect.
Depending on their application, pressure vessels in general must comply with a set of high performance requirements, including structural rigidity under high pressure and the ability to provide good thermal insulation, corrosion resistance and chemical compatibility with the stored contents of the vessel. One common solution to satisfying these requirements is to make the vessel as a composite over-wrapped pressure vessel (COPV), formed by wrapping a relatively thin metal tank with a composite material to provide the required strength and thermal insulation properties without adding significant additional weight to the structure. The metal tank serves as an impervious liner to the composite outer layers of the vessel and may be joined to integral metal bosses with flanges for coupling fluid lines to the vessel. Composite over-wrapping is facilitated, in the case of a cylindrical vessel, by rotating the metal tank on its longitudinal axis to achieve the application of the composite material. Typically, overwrapping uses fibers or strands of carbon (or similar material such as Kevlar® or fiberglass) wrapped around the vessel in a continuous process and using a suitable matrix.
Although composite over-wrapped pressure vessels are perfectly satisfactory for many applications, they have significant drawbacks for aerospace applications, such as in launch vehicles and for spacecraft in general. When used in aerospace applications to store fuel or other substances, pressure vessels must not only have the required strength and rigidity of pressure vessels used on the ground, but they must also be as light as possible because a key limitation in spacecraft design is the cost to launch a desired mass into orbit about the Earth, or beyond. Another requirement for aerospace applications of pressure vessels is that they must be as immune as possible to changes in temperature. Unfortunately, common composites such as carbon composites have a lower coefficient of thermal expansion (CTE) than most metals. Compounding this problem, metals with a lower CTE, such as Invar® or Monel®, are much more dense than common metals such as steel or aluminum, thus increasing the overall weight of the pressure vessel if any of these lower CTE metals are used.
It has long been recognized, therefore, that the ideal form of construction of pressure vessels for aerospace and other applications is one that employs only composite materials to form a seamless vessel, without the use of a metal liner or other metal components. In the conventional composite over-wrap pressure vessel (COPV) manufacturing process, a steel liner tank is used as a mandrel for over-wrapping with a composite material. An alternative process employs a soluble, usually water-soluble, mandrel that can be flushed out of the vessel at the end of the manufacturing process. The process is sometimes referred to as the wash-out mandrel process. Unfortunately, the material used to form such a mandrel is relatively fragile in its solid form and until now has been used only to make quite small vessels. One example of such material is AQUAPOUR®, manufactured by Advanced Ceramics Research, Inc., Tucson, Ariz. The water-soluble mandrel material has physical properties similar to those of plaster of Paris, which is very useful as a finishing or repair material but has only limited weight bearing ability. For larger pressure vessels such as those required for fuel storage in aerospace applications, a conventionally formed water-soluble mandrel cannot long withstand the force of gravity acting on the mass of the mandrel itself, as when mounted on a horizontal axis for over-wrapping. Also, the forces applied to the mandrel in the over-wrapping process are large enough to distort or break a mandrel made from soluble materials in a conventional way.
For the foregoing reasons, soluble mandrels have not been used to make pressure vessels greater than a few inches in diameter and length. Mandrels any larger than this are so fragile that they easily break during handling or lay-up of the composite material, or simply break under their own weight when mounted on a horizontal axis. Extremely large vessels, and in particular those that are large enough for workers to enter through an opening, can be formed by over-wrapping a rigid mandrel that is capable of disassembly from inside the pressure vessel. There is, however, a large class of pressure vessels that are smaller than this but are large enough that soluble mandrels cannot be conventionally used without significant risk of mandrel breakage during the process.
A known technique for fabricating composite pressure vessels is to make a larger vessel of several segments of lesser length, and then to join the segments by further over-wrapping them at each interface between segments. The joint over-wrapping material is usually referred to as a belly band. Unfortunately, all belly band approaches create opportunities for leaks to occur along the joint line. Therefore, they are not a favored form of construction in applications where the integrity of the pressure vessel is a top concern.
From the foregoing description of the art of composite over-wrapping, it will be appreciated that for the many years since space travel has become a reality, there has been a great need to provide pressure vessels that are lightweight but without compromise in strength, rigidity and integrity at high pressures. The present invention in accordance with one of its aspects achieves this goal.
A related drawback of pressure vessels of the prior art is that they typically utilize a metal boss at one end or both ends of the pressure vessel. The metal boss typically has a female thread formed in it, although it may in some cases have a male thread or use some other way of connecting with a fluid-carrying pipe. The metal boss is typically integrated with a metal mounting flange that has to be integrated into the composite material of the pressure vessel. Just as it is desirable to reduce the use of metal in the vessel itself, it is also a goal to reduce the use of metal in bosses and flanges used for coupling feed pipes to the vessel. In smaller tanks, of course, the additional weight of a boss and associated flange is not highly significant but in larger tanks the goal of weight reduction ideally requires the avoidance of metal in bosses and flanges as well as in the tank itself. Metal flanges pose an additional problem because of the CTE differential, mentioned earlier, between most common metals and composite materials. The ends of a pressure vessel, where inlet and outlet ports are typically installed, are inherently weak points in the vessel and the presence of expansion differentials can lead to cracks and, ultimately, to pressure failure.
It has long been a goal of designers of large pressure vessels to eliminate metal materials entirely, even from the boss and flange components that are conventionally formed in metal. Achieving this goal, however, has eluded pressure vessel designers for decades. One problem is that successive layers of composite material have a relatively low degree of adhesion between them. Merely stacking layers together can lead to de-lamination when stress is applied during the lay-up process or later. In addition, the laid-up fabric around a boss and flange must be applied both in a radial direction and in a longitudinal direction. Finally, it would seem that cutting a thread into the boss would necessarily cut through fibers in the composite and thereby destroy the structural integrity of the boss. The present invention achieves the goal of using composite materials for each threaded (or non-threaded) boss and flange, resulting in a unitary composite pressure vessel with no metal parts.
Another related requirement for pressure vessels in many applications is that they include integral “stringers” that extend longitudinally or circumferentially on the outside of the vessel. In some contexts, stringers are members that strengthen or stiffen a structure, such as a hull or fuselage. In the present context of pressure vessels, longitudinal and circumferential members are used not only as stiffeners but also to facilitate handling and installation of the vessel and to facilitate attachment of other associated components, or attachment of the pressure vessel itself to a structure in which it is to be installed. In the past, stringers have been superficially attached by bonding them to a completed composite over-wrapped pressure vessel (COPV). Consequently, there is always a risk of separation of the stringer or de-lamination of the vessel surface when a stringer is subject to stress. Complete integration of longitudinal and circumferential stringers is another aspect of COPV construction that has eluded designers prior to the present invention.
It will be appreciated from the foregoing that there has been a very long felt need for improvements in processes for fabrication of composite pressure vessels. In particular there has been a need for a process that allows for the production of very large pressure vessels without metal liners, as well as for a process that allows the integration of composite coupling bosses and associated flanges into a single unitary pressure vessel, and integration of composite stringers into the same unitary structure. The present invention accomplishes all of these goals, as will be apparent from the following summary.